Dispose the v8object handle in QQmlData::destroy()

Dispose the v8object handle in QQmlData::destroy()

Previously, the handle was simply cleared, which meant that the data
referenced by the handle would only be destroyed during the next
garbage collection cycle.  That had the unwanted side effect that
v8's gc heuristics would assume that the old pointer space needed
expansion more regularly than is required in truth, which has the
further effect of slowing down mark-sweep-compact cycles considerably.

Change-Id: I41595a4d79a75866f434c371139e5493e757d6ab
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>